Limb and orofacial stiffness is modulated by central descending neural inputs to lower motor neurons, reflex gain, muscle and connective tissue properties, postural orientation to gravitational loads, and geometry of muscle attachments. A common approach for quantifying muscle stiffness involves imposing a specific displacement (ΔX) on a muscle-tissue system (i.e., whole body, limb, jaw) and measuring the resultant force (ΔF). The ratio of the resultant force to displacement yields a stiffness quotient (ΔF/ΔX). Muscle rigidity, regarded as the clinical correlate of stiffness, is used as a diagnostic method to evaluate neurologic status, document the effects of disease progression, pharmacological efficacy, and neurosurgical intervention.
Although biomechanical studies of limb rigidity have provided valuable insight into the neural regulation of limb movement disorders, similar application to orofacial systems has been tenuous primarily due to inadequate methods of transduction for the ‘floating’ perioral tissue complex. Limb muscles typically have tendonous attachments to bone, and utilize a combination of muscle spindle receptors and Golgi tendon organs to regulate stiffness about joints for postural and voluntary movements. The anatomical organization is much different for the lower face. Most perioral muscles have insertions into the integument of skin, interdigitate with neighboring muscle groups, and lack classic muscle spindle-tendon organs. Significant differences in mechanoreceptor representation are apparent between hand and orofacial systems. The neural mechanisms underlying stiffness regulation for the face and limb are likely to differ in their expression in health and disease.
Therefore, there is a need for improved devices and systems for measuring orofacial stiffness. It would be beneficial to be capable of obtaining real-time perioral stiffness measurements in unrestrained participants in health and disease. It would be advantageous to be able to perform passive ‘non-participatory’ perioral stiffness to provide clinicians a new techniques for evaluating efficacy of pharmacological and surgical intervention in the perioral system of patients with neuromotor disease, craniofacial anomalies (i.e., cleft lip), or traumatic injury (i.e., bomb blast, missile wounds, vehicular).